Multilayer ceramic capacitor

ABSTRACT

The multilayer ceramic capacitor has a plurality of dielectric layers and inner layers laminated alternately laminated on one another. In each of the dielectric layers, the rate of ceramic grain which form the dielectric layers and present singly in one dielectric layer over its entire longitudinal thickness is set to amount to 20% or more. The multilayer ceramic capacitor can prevent a decrease in a CR product to a value lower than a desired level even if the dielectric layer becomes as thin as 5 μm or less. This can comply with demands for a multilayer ceramic capacitor that the number of the dielectric layers to be laminated on the inner electrodes should be increased yet the thickness of the dielectric layers should be made thinner in order to meet requirements for making electronic circuit more compact in size and higher in density.

CROSS-REFERENCE

This application claims the priority of Japanese Patent Application No.H10-121,997, filed on May 1, 1998, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilayer ceramic capacitor whichhas a large number of laminated layers of dielectric layers and innerelectrodes, whose dielectric layers are each set to be thin, and whichis compact in size yet large in capacitance.

2. Description of the Related Art

A multilayer ceramic capacitor is composed of a chip-shaped elementassembly and a pair of outer electrodes which are formed at both endportions of the element assembly. The element assembly is in turncomposed of a number of laminated layers in which a number of dielectriclayers and inner electrodes are alternately laminated on one another.The inner electrodes are configured such that the adjacent electrodesare disposed so as to face each other with the dielectric layer betweenand to electrically connect them individually to the outer electrodes.

The element assembly may be produced by laminating ceramic green sheetsand conductive patterns alternately on one another to form a laminatedassembly in a chip shape and firing the chip-shaped laminated assemblyat a high temperature in the range of approximately 1,200° C. to 1,300°C. in air.

The ceramic green sheet to be used for conventional laminated ceramiccapacitors is composed of ceramic grain of, e.g., BaTiO₃ type, etc.,which has a high dielectric constant, and an organic binder as maincomponents. On the other hand, the conductive pattern may be composed ofa conductive paste containing powders of, e.g., Pd or the like as a maincomponent.

The component such as Pd or the like to be used for the main componentof the conductive paste is a noble metal which is expensive. Therefore,conventional multilayer ceramic capacitors are expensive in productioncosts. Recently, a base metal such as Ni or the like has been used asthe main component for the conductive paste in order to reduceproduction costs of ceramic capacitors.

However, the use of the base metal such as Ni, etc., as the maincomponent of the conductive paste for the production of chip-shapedlaminated assembly may suffer from the disadvantages that the such basemetal is caused to undergo oxidation, when it is burned at hightemperature in air in a conventional way, and as a result that theconductivity of the inner electrodes may be lost. On the other hand,when the chip-shaped laminated assembly is burned at high temperature ina non-oxidative atmosphere in order for the base metal such as Ni or thelike to fail to undergo oxidation, the dielectric layers are caused tobe reduced, thereby lowering resistance to insulation and as aconsequence failing to gain a desired level of electricalcharacteristics.

Therefore, when the base metal such as, e.g., Ni or the like is used asa material for the inner electrode, a desired level of electricalcharacteristics is designed to be achieved by using a material having ahigh degree of resistance to reduction for the dielectric layer. In thiscase, the firing is first carried out in a reductive atmosphere and thethermal treatment is then carried out at temperature lower than thefiring temperature, i.e. at approximately 600° C. to 900° C., in anatmosphere containing a small amount of oxygen to re-oxidize thedielectric layer and recover the resistance to insulation of thedielectric layer, while preventing the inner electrode from undergoingoxidation.

Recently, there are growing demands to make multilayer ceramiccapacitors compact in size and larger in capacitance in order to complywith requirements for electric circuits which have smaller in size yethigher in density. Therefore, attempts have been made to furtherincrease a number of laminated dielectric layers and make them thinner,in order to meet such requirements for making the multilayer ceramiccapacitor compact in size and larger in capacitance.

It has to be noted, however, that the further increase of the number ofthe multilayer dielectric layers and the thinning of them can improve acapacitance of the resulting multilayer ceramic capacitor, but they maycause a decrease in resistance to insulation of the resulting multilayerceramic capacitor. The reason for the decrease in resistance toinsulation is because the insulation resistance R can be represented byR=(ρ×d)/S, where R is resistance, ρ is resistivity, S is the area of theelectrode, and d is the thickness of the dielectric layer.

Further, a CR product can be enumerated as one item for characteristicsof a capacitor. The CR product is the product obtained by multiplyingthe capacitance C by the insulation resistance R. Thus, as thecapacitance C can be represented as C=εo×εr×S/d, wherein εo is adielectric constant in vacuo; and εr is a specific dielectric constant,the CR product can be represented by C×R=ρ×εo×εr. As a result, it can befound that the CR product can give a value that does not depend upon thethickness of the dielectric layer and the number of the layers thereof.

It should be noted, however, that, generally speaking, the CR productshows the tendency that it may decrease when the thickness of thedielectric layer becomes thinner than 5 μm. This is considered to occurbecause the resistance to insulation is caused to deviate from the ohmicrule as the dielectric layer becomes thinner. In other words, ceramiccapacitors may cause the problem that the CR product may be reduced asthe ceramic capacitor becomes thinner.

SUMMARY OF THE INVENTION

Therefore, the present invention has an object to provide a multilayerceramic capacitor which is compact in size and is provided with a highcapacitance and which produces a desired level of the CR product, evenif a dielectric layer is made thinner.

In order to achieve the object, the present invention provides amultilayer ceramic capacitor having a number of dielectric layers andinner electrodes laminated alternately on one another, in which a rateof single ceramic grain present in one dielectric layer amounts to 20%or higher.

The other objects, features and advantages of the present invention willbecome apparent in the course of the description of this applicationwith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an example of a firing pattern of a multilayerceramic capacitor according to an embodiment of the present invention.

FIG. 2 is a schematic illustration showing a sequence of ceramic grainin a section of a dielectric layer of the multilayer ceramic capacitoraccording to the embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The multilayer ceramic capacitor according to the present inventioncomprises a large number of laminated layers of dielectric layers andinner electrodes, which are laminated alternately, whose dielectriclayers are each set to be thin, and which is compact in size yet largein capacitance.

In the multilayer ceramic capacitor according to the present invention,each of the dielectric layers is configured such that single ceramicgrain present in one dielectric layer at a rate of 20% or more.

The dielectric layer may be produced from a material of, for example, aBaTiO₃ type, although a material other than the BaTiO₃ type material canalso be used.

Further, each of the dielectric layers may preferably have a layerthickness in the range of 5 μm or thinner, although it may have a layerthickness of thicker than 5 μm. Moreover, the ceramic grain maypreferably have a mean grain size of 3.5 μm or larger, although thegrain sizes of the ceramics depend upon the thickness of the dielectriclayer. If the thickness of the dielectric layer is thicker than 5 μm,the CR product of the resulting multilayer ceramic capacitor may bedecreased to a remarkable extent.

The inner electrode may be produced by firing a conductive pastecontaining Ni powders as a major component, although it may be producedfrom a conductive paste containing a base metal other than Ni as a majorcomponent.

The rate of the single ceramic grain present in one dielectric layer, asreferred to in this description, can be determined, for instance, bycutting the multilayer ceramic capacitor along the longitudinal planeperpendicular to the inner electrodes and measuring the grain sizes ofthe ceramics present in the dielectric layers appearing in the cutsection. The mean grain size of the ceramics is then calculated.Further, a predetermined number of lines are drawn in the longitudinaldirection perpendicular to the inner electrodes over the entiresectional area of the multilayer ceramic capacitor each at an intervalof the mean grain size. Moreover, the number of the lines are countedwhich cross each a single ceramic grain present in one dielectric layer,which is located singly over its entire longitudinal region of thedielectric layer in the position where the line is crossing. Thereafter,the rate in percentage of the such single ceramic grain that are presentsingly in one dielectric layer in the direction perpendicular to theinner electrodes is calculated by dividing the lines passing through thesuch single ceramic grain by all the predetermined number of the linesdrawn longitudinally and perpendicularly to the inner electrodes andthen by multiplying the resulting quotient by 100.

The present invention will be described in more detail by way ofexamples with reference to the accompanying drawings.

EXAMPLE

A ceramic green sheet was prepared in a conventional way and aconductive pattern was printed on the resulting ceramic green sheet. Aplural number of the ceramic green sheets so prepared are laminated onone another and pressed into a laminated sheet. The resulting laminatedsheet was cut into chip-shaped pieces so as for each to contain theconductive pattern.

In this example, the ceramic green sheet was prepared from ceramicpowders of BaTiO₃ type. The conductive pattern was formed by using aconductive paste containing Ni powders as a major component. Further,the ceramic green sheet was prepared so as to be as thick as 5 μm afterfiring.

As shown in FIG. 1, the chip-shaped laminated members were heated in airby raising the temperature of a furnace up to 600° C. to burn out thebinder, followed by turning the atmosphere of the furnace into anon-oxidative atmosphere consisting of nitrogen gas containing 2.0% byvolume of H₂ and elevating the temperature to 1,200° C. to 1,300° C.Further, they were fired at the same temperature for 1 to 5 hours, andthe temperature of the furnace was lowered to 600° C. Then, thenon-oxidative atmosphere of the furnace was changed into a nitrogen-gasatmosphere containing 200 ppm of oxygen, and they were heated at 600° C.for another 1 hour to re-oxidize the dielectric layers. Thereafter, thetemperature of the furnace was allowed to cool to ambient temperature.The grain sizes were changed by varying the firing temperature andresidence time.

Then, the resulting laminated members were polished after firing,followed by heating them at 1,150° C. to 1,200° C. to subject them tothermal etching. An SEM image of the polished surface was taken at the2,000-fold magnification. On the other hand, the grain sizes of 200grain were determined by measuring the diameter of each of the grain,which extends in the direction parallel to the inner electrodes. Then,the mean grain size of the 200, grain was calculated. The results areshown in Table below.

On the other hand, 100 lines were drawn longitudinally each at theinterval equal to the mean grain size calculated above on the abovemicroscopic picture in the direction perpendicular to the innerelectrodes. The number of the lines were then counted on the microscopicpicture, each of which crosses only one single grain present in onedielectric layer over its entire length of the line. The such linescrossing the such single grain were further divided by the 100 lines asoriginally drawn, and represented by percentage as a rate of singlegrain present in one layer. More specifically, the “single grain presentin one layer” referred to herein indicate single grain A and B each ofwhich is present singly in one dielectric layer 12 interposed betweeninner electrodes 10 and 10, as shown as grain “A” and “B” in FIG. 2,respectively.

Then, the chip so prepared was burned to sinter it, and the resultinglaminated member was provided with outer electrodes at its both ends bybaking to form a multilayer ceramic capacitor. The multilayer ceramiccapacitor was then measured for capacitance with an LCR meter by placingit at 20° C. in a chamber. The measurement was carried out underconditions at 1 kHz and 1 Vrms. After measurement of the capacitance,the insulation resistance was measured by applying DC 50 volts to themultilayer ceramic capacitor for 1 minute. The capacitance and theinsulation resistance were multiplied to give a CR product asrepresented in μF·MΩ. The results are shown in Table below.

TABLE SAM- MEAN PARTICLE SIZE RATE OF SINGLE CR PLE (μm) OF CERAMICGRAIN PRESENT IN PRODUCT NOS. GRAIN ONE LAYER (%) (μF · MΩ) 1 1.2  0 3,000 2 2.0  0  3,200 3 2.9  3  6,000 4 3.2 10  8,500 5 3.5 20 15,000 63.5 35 15,500 7 3.5 45 16,800 8 4.0 53 18,700 9 4.2 60 20,000 10  5.0100  17,000

From Table above, it can be found that a CR product of 15,000 μF·MΩ orhigher can be achieved when the rate of single grain present in onelayer amounts to 20% or higher.

EFFECTS OF THE INVENTION

For the multilayer ceramic capacitor according to the present invention,the reductive property can be suppressed at the stage of firing becausethe surface area of the ceramic grain with the dielectric layers formedthereon becomes small. Further, the thinning of the thickness of themultilayer ceramic capacitor can further make it likely to disperseoxygen in the grain boundary with ease after the oxygen has dispersedthe interface between the inner electrodes and the ceramic grain at thestage of re-oxidation. Therefore, it is considered that the ohmicresistance characteristics can be maintained. In other words, making thethickness of the multilayer ceramic capacitor thinner can realize thesefeatures more effectively.

With these features, the present invention can achieve the effect thatthe multilayer ceramic capacitor can be provided with a large amount ofcapacitance, even if the dielectric layer is thin, without causing adecrease in the CR product even in a region where the CR product isotherwise caused to decrease in usual circumstances, although it can bemade compact in size.

What is claimed is:
 1. A multilayer ceramic capacitor having a pluralityof dielectric layers and inner electrodes laminated alternately on oneanother, wherein a rate of single ceramic grain present in onedielectric layer amounts to 20% or higher, said single ceramic grainbeing a ceramic grain each of which is located singly in one dielectriclayer over an entire longitudinal length of said one dielectric layer.2. The multilayer ceramic capacitor as claimed in claim 1, wherein: saiddielectric layer is as thin as 5 μm or less, and said ceramic grainforming said dielectric layer has a mean grain size of 3.5 μm or larger.3. The multilayer ceramic capacitor as claimed in claim 1, wherein saidinner electrode is formed by a conductive paste containing Ni powders asa major component.
 4. The multilayer ceramic capacitor as claimed inclaim 1, wherein said dielectric layer comprises ceramic grain of aBaTiO4 type.
 5. The multilayer ceramic capacitor as claimed in claim 2,wherein said inner electrode is formed by a conductive paste containingNi powders as a major component.
 6. The multilayer ceramic capacitor asclaimed in claim 2, wherein said dielectric layer comprises ceramicgrain of a BaTiO4 type.